In forming polymeric plastic articles by injection or co-injection molding processes, it is often desirable to produce a plastic article having an interior core layer of a first polymeric material and a skin of at least one other polymeric material. The interior core layer is formed from an interior core stream of the first polymeric material. The skin, which has an inner layer and an outer layer, is formed from an inner and an outer stream of at least one other polymeric material. The same polymeric material(s) may be used for both the inner stream/inner layer and the outer stream/outer layer, or different polymeric materials may be used for the inner stream/inner layer than for the outer stream/outer layer.
The interior core stream is combined with the inner and outer streams to form an annular combined polymeric stream. Various methods of combining the polymeric streams may be employed. For example, the combined polymeric stream may be formed by sandwiching an annular interior core stream with an annular inner stream and an annular outer stream in an apparatus used to inject the combined stream into a mold cavity. The combined stream is then forced along the annular pathways of the mold cavity to form a molded plastic article having desired dimensions (i.e., configuration, thickness, etc.).
The interior core stream material is often selected such that the interior core stream forms an interior core layer in the resulting plastic article that functions, in some instances, as a barrier layer or as a scavenger layer. The barrier layer prevents foreign substances from invading the environment enclosed by the plastic article and/or prevents substances within the plastic article from escaping to the outside environment. One example of a scavenger layer is an oxygen scavenger layer
In some situations, the barrier material and the skin materials do not adhere to each other without adding an adhesive material to the skin material, to the barrier material, or to both. Accordingly, in order to improve the adhesion of the interior core layer and the skin layers, adhesives are generally added before or during the molding process. For example, in some instances, an adhesive is added to the polymeric material that forms the skin, to the polymeric material that forms the interior layer, or to both.